This invention relates to an electronically controlled fluid suspension system for preventing a vehicle from laterally rolling.
In order to enhance riding comfort and steering stability, various suspension systems have been proposed. For example, in a system disclosed in Japan published unexamined utility model application No. 60-152510, the normal cornering and the rapid cornering of the vehicle--depending on rapidity of steering--are detected distinctively from each other by determining whether or not the steering angle speed is not less than a standard value. If the normal cornering is thus detected, the compressed air is supplied by a reduced amount and the vehicle attitude is controlled in a relatively gradual manner according to the controlling time period calculated by a map representing a relation between a steering angle and a vehicle speed. If rapid cornering is detected, on the other hand, the compressed air is supplied by an increased amount so as to control the vehicle attitude rapidly according to the aforementioned map.
Yet, this prior art electronically controlled suspension system has some problems. Specifically, in such a system that detects the steering angle and the steering speed, the actuators and other devices are controlled to be actuated corresponding to the values of steering angle or speed which are detected every moment. As a result, the actuators and other devices are operated rather frequently, thus causing a problem in realizing an immediate response of the actuators and other devices. If actuation is controlled according to the average of the detected values of the steering speed, etc., the above problem as to the operational frequency and responsiveness may be solved. However, in case of a rapid steering immediately followed by a reverse rapid steering, the average value becomes small, resulting in that the vehicle attitude is controlled not rapidly but gradually. Therefore, the control of the vehicle attitude is occasionally delayed.
There are other prior arts which control the damping force of a shock absorber or the spring constant of an air spring. For example, in a system disclosed in Japan published unexamined utility model application No. 60-119608, the damping force of the shock absorber and the spring constant of the air spring are controlled upon detection of a predetermined lateral acceleration so as to prevent rolling, thus keeping the vehicle body horizontal. In another system disclosed in Japan published unexamined utility model application No. 60-119621, the spring constant of the air spring is controlled based on the steering angle speed of the steering wheel and the vehicle speed. Corresponding to the steering angle speed or the lateral acceleration detected at the beginning of cornering, these prior art electronic controlled suspension systems predict the subsequent roll of the vehicle body and control the spring constant. In this case, the predicted roll is occasionally different from the actual roll because of the external disturbance such as different frictional coefficients on an uneven road surface or a cross wind. Therefore, the aforementioned prior art suspension systems cannot satisfactorily improve riding comfort or steering stability.